Back-drilled holes have been increasingly widely applied in the existing industry of Printed Circuit Boards (PCBs), and the back-drilled holes are an important factor in a design of PCBs particularly for existing electronic products transmitting signals at a high speed.
When fabricating a PCB, through-holes need to be set for connection between respective inner layers of lines, where copper deposition, electroplating, and other processes need to be further performed on the through-holes to form electrically conductive layers in the through-holes to thereby connect the respective layers of lines. However through-holes of some PCBs need to be only partially conducting, whereas the through-holes on which copper deposition and electroplating are performed are completely conducting, so that there may be such a problem arising with connected ends of the through-holes that a signal may travel back so that the signal being transmitted may be reflected, scattered, delayed, etc., so the signal may be “distorted”. In order to prevent this from arising, the through-holes need to be back drilled.
Along with constant progress of the electronic products, there has been an increasingly stringent requirement on a back-drilling process for the design of the electronic products, and particularly if small holes (with an aperture at or below 0.25 millimeter) for the design of the high-speed electronic products are back drilled, then it is required that no copper wire can remain in the back-drilled small holes.
In view of the requirement above, a practice in the industry to deal with the copper wires remained in the back-drilled small holes is to design an alkaline etching flow process. FIG. 1a to FIG. 1i are schematic structural diagrams of respective operations in a flow of a method for fabricating back-drilled holes on a PCB in the prior art, and as illustrated in FIG. 1a to FIG. 1i, the alkaline etching flow process generally includes opening a material, transferring an inner pattern, browning, laminating an outer board to form a PCB 01 (as illustrated in FIG. 1a), drilling 011 (as illustrated in FIG. 1b), depositing copper and pre-plating the board (the thickness of the copper 02 in the holes is controlled ranging from 5 micrometer to 8 micrometer, and illustrated in FIG. 1c as a thickened layer in the holes, although the thickened part of surface copper is not illustrated), placing a dry film 03, transferring an outer alkaline pattern (as illustrated in FIG. 1d), electro-plating the pattern (the thickness of the copper in the respective holes is sufficiently plated as required by a customer, as illustrated in FIG. 1e), plating tin 04 at positions where lines are windowed (as illustrated in FIG. 1f), back-drilling (as illustrated in FIG. 1g), removing the dry film (as illustrated in FIG. 1h), alkaline etching (as illustrated in FIG. 1i), etc.
However although the problem of the copper wires remained in the back-drilled small holes can be addressed in the alkaline etching flow process above, there may be numerous drawbacks generally including the following items:
1. The alkaline etching flow process designed for back-drilling the small holes is very complicated and comes with a long production flow, thus resulting in low productivity.
2. There is a very stringent requirement on the thickness of the base copper on the PCB in alkaline etching, where generally the thickness of the base copper needs to be controlled within 50 micrometers; otherwise, fine lines (with a line width generally no more than 4 mils, where 1 mill is approximately 25.4 micrometers) might fail to be etched as required.
3. Surface copper needs to be plated twice in the alkaline etching flow process designed for back-drilling the small holes, so that the surface copper may easily become excessive in thickness, and the small holes may become smaller in diameter, making it very difficult to plate tin in the small holes, so there may be no sufficient tin in the holes to protect the hole copper, so that the copper which should not be etched may be etched away in subsequent etching for removing the copper wires remained in the back-drilled holes, and thus the small holes may easily be rejected due to the absence of copper therein.